I believe that Great Plains specifies 30 W, not multiviscosity, for the VW engine. I drove air-cooled VW's for about 500,000 miles in the 70's, and I never had a problem with the standard 30W that VW specified. However, the oil does work harder in an air-cooled engine, so it should be changed more often. My rule of thumb was to feel the oil on the dip stick. When it felt gritty, it was time to change. That was as little as 1200 miles during hot weather, but never over 3000 miles in cold weather.
I'm not sure what the Corvair specification is, but I would suggest considering whatever is the standard oil for a Corvair car engine. Jim Vance va...@claflinwildcats.com
